1. Field of the Invention
The present invention is directed to a low-profile dome-shaped multi-lens system, more particularly to such a system for collecting infrared radiation from a panoramic field of view to an optical detector for motion detection.
2. Description of the Prior Art
U.S. Pat. No. 4,930,864 discloses a prior lens system covering a panoramic field of view for infrared motion detection within a wide detection range. The system comprises a semispherical shell provided with a plurality of convex lenses arranged to define a plurality of sensitive areas in the panoramic field of view for collecting infrared radiation from each sensitive area to an optical detector. Upon receiving infrared radiation from persons or objects passing through the sensitive areas, the optical detector provides a signal of fluctuating radiation level which is processed by an associated electronic circuitry for detecting a motion of the persons or objects. This lens system is designed to give an equal focal length to all the lenses and to dispose the optical detector at a spherical center of the semispherical shell through which the optical axes of all the lenses pass. The lens system is capable of collecting infrared radiation from the discrete sensitive areas distributed over a wide solid angle ranging up to about 2.pi. steradians. However, in this prior lens system, the peripheral lens disposed at a greater angular disposition about the center axis defines the peripheral sensitive areas of correspondingly larger size so as to give a difference in the size of the sensitive areas defined on a horizontal plane, i.e., a floor of a room when the system is mounted on a ceiling. The difference becomes more critical as the sensitive areas are distributed over a wider angular range, which makes it difficult to determine a precise motion detection at the peripheral sensitive areas as is possible for the center sensitive areas. It is therefore demanded to provide the sensitive areas of even size over a wide angular range in order to give a consistent motion detection over the widely distributed sensitive areas. Further, it is also desired for aesthetic reasons to reduce an exposure height measured along a center axis of the semispherical shell from the vertex thereof to the outermost lens, i.e., peripheral one disposed at a maximum angle about the center axis, provided that the remaining portion of the system can be concealed within the ceiling.